Rare earth complexes show sharp luminescence based on 4f-4f electronic transition, and thus recently attract the attention as novel luminescent materials. In addition, when rare earth complexes comprises organic ligands as a constituent, they exhibit compatibility with polymers and the like, and thus are proposed to be mixed with polymer materials and used as light-emitting devices or fluorescent inks. The rare earth complexes are also known to exhibit high heat resistance (PTL 1 and 2 and NPL 1).
Further, complex polymers which are polymerised due to a large number of coordination sites of complexes attract the attention as novel light-emitting materials. It has been reported that formation of coordination polymers by combining Eu(III) ions and aryl units can attain an improved thermostability and high emission quantum yield (NPL 2).    [PTL 1] WO 2012/15072    [PTL 2] Japanese Patent No. 3668966PTL 1 and PTL 2 are expressly incorporated herein by reference in their entirety.    [NPL 1] K. Miyama, Y. Hasegawa et al, hem. Eur. J., 2011, 17, 521-528    [NPL 2] K. Miyata, T. Ohba et al, Chem Plus Chem., 2012, 77, 277NPL 1 and NPL 2 are expressly incorporated herein by reference in their entirety